Summary
Fastsore uses the Intelligent search to load the PDP, but it doesn't use the regionID to load it on regionalized stores. This leads to problems with the items' availability. Also, we use the hideUnavailableItems=true prop on the query to load the PDP. This can lead to inconsistencies between the PLP (which is regionalized) and the PDP.
Simulation
Try to look for a product that has stock in the regionID that you are regionalized in. If this product doesn't have stock on the comprehensive seller and only on specific sellers unrelated to this regionID, the PLP and the PDP may have different information.
Workaround
Enable "ShowIfNotAvailable" in the products via Catalog to minimize the effects of this problem. Add the seller as comprehensive so the product will be returned